The present invention relates to a radio unit for a mobile station used in mobile communications and more particularly, to a portable radio unit.
Among conventional radio units for mobile stations used in mobile communications, the portable radio unit carried by a user has been provided with a built-in retractable antenna with its length shortened by .lambda./2 or .lambda./4. Such a radio unit, for example, a radio phone system, is required to keep its antenna fully extended as much as the unit is allowed to the limit for keeping good receiving sensitivity during standby and communication periods.
Carrying the portable radio unit leaving its antenna fully extended is generally inconvenient for a user. So it is likely to be carried in a bag or in a pocket with its antenna retracted. Assuming that there is a call for the user while he/she is carrying it with its antenna retracted, communication between a base station of control signal for connecting the call and the destination radio unit is executed with the antenna left retracted. When the antenna is retracted, its gain is lowered. In case the user stands in a place of low electric field of transmission wave from the base station, it is likely to cause receiving failure, resulting in deteriorating receiving call rate. In case of giving a call by the radio unit with its antenna retracted may also deteriorate connection rate.
A means for enhancing the receiving sensitivity while the antenna of the portable radio unit is kept retracted has been disclosed in Japanese Patent Laid-Open No. 252326(1990). This Japanese Patent Laid Open No. 252326 (1990) relates to a cordless phone system provided with a base set and a hand set. In the above art, an electric wave between the base and hand sets may be selected to be the one with excellent S/N ratio (power ratio of signal to noise), for example, FSK, ASK, PSK, and MSK during standby period other than communication period. The receiving band of the hand set is further decreased to improve the S/N ratio in a weak electric field.
This system, however, requires a modulation means of a different electric wave mode such as SSB and FM, which further needs specific circuit construction, resulting in increased manufacturing cost.